How about an episode where the ship is hit with a gender-o-beam, and everyone temporarily switches genders?
... Yeah, that would only have had a 96% chance of being completely terrible.
We already had the EMH and 7 switch.
Would the have just turned into the other gender, like BAM! Janeway is suddenly a man or would they switch bodies with crewmen. The switching with crewmen thing has been done in so many other shows but I guess they don't just BAM! change genders because it's going to be costly.